1. Field of the Invention
The present invention relates to an apparatus and a method for measuring a living body, and more particularly, to an apparatus and a method for measuring living body composition, such as skin and subcutaneous fat, using light.
2. Description of the Related Art
A number of different methods for measuring body fat percentage or subcutaneous fat thickness are known. For example, methods using calipers, ultrasound, computer tomography (CT), magnetic resonance imaging (MRI) are some of the known methods for measuring subcutaneous fat thickness. However, measurement with calipers often may be inaccurate, inconvenient, and cause pain to a person receiving the measurement. Other measurement methods involving use of ultrasound, CT, or MRI require use of expensive medical imaging machines that are operated by professionals who are trained to use the machines. In addition, such measurement machines are not portable, and cannot be used readily at a desired place or time. As a solution to these problems, various living body measurement methods using light have been proposed.
According to the proposed methods using light, body fat percentage or subcutaneous fat thickness is measured by radiating light onto the surface of the skin and detecting the light emitted from the surface of the skin by multiple scattering. Such measurement is non-invasive, and takes a short time. Since a small portable apparatus is generally designed for that measurement, it can be used wherever and whenever necessary.
FIG. 1 is a view illustrating a conventional apparatus for measuring subcutaneous fat thickness. A living body tissue 130 has a structure made up of muscle 132, fat 134 and skin 136 with thickness ranging from 0.5 to 4 mm. The skin 136 is divided into stratum corneum, epidermis and dermis. The apparatus 100 for measuring subcutaneous fat thickness includes a light emitting diode (“LED”) 110 and a photodiode (“PD”) 120. The LED 110 and the PD 120 are separated from each other and placed on the top surface of the skin 136. The LED 110 emits light in near-infrared bandwidth onto the surface of the skin 136. A portion of the light traveling from the surface of the skin 136 to the muscle 132 is reflected back to the skin 136 by multiple scattering, while rest is absorbed by the muscle 132. The PD 120 detects the light emitted from the surface of the skin 136 as an electric signal, after converting the emitted light to an electrical signal.
FIG. 2 is a graph showing output variations of the PD 120 according to the distance from the LED 110. The transverse axis of the graph represents the thickness, in mm, of the subcutaneous fat 134, whereas the longitudinal axis represents the output voltage V of the PD 120. The term “SD” refers to the distance between the LED 110 and the PD 120. FIG. 2 depicts points plotted on curves that map subcutaneous fat thickness to the corresponding output voltage in SDs of 5 mm, 10 mm, and 20 mm. The graph in FIG. 2 shows that the output of the PD 120 gradually increases with an increase of the thickness of the subcutaneous fat 134, but does not increase further from a specific thickness level. From this graph, it is clear that greater the distance SD, broader range of subcutaneous fat thickness may be measured.
U.S. Pat. No. 4,850,365 titled “Near Infrared Apparatus and Method for Determining Percent Fat in a Body” and issued to Rosenthal et al. discloses a method and an apparatus for measuring body fat percentage by transmitting near-infrared radiation at only one wavelength into the skin and detecting the light emitted from the surface of the skin due to multiple scattering within the subcutaneous fat.
U.S. Pat. No. 4,633,087 titled “Near Infrared Apparatus for Measurement of Organic Constituents of Material” and issued to Rosenthal et al. discloses a technique for measuring body fat percentage by transmitting near-infrared radiation of different wavelengths into the skin and detecting the light emitted from the surface of the skin due to multiple scattering within the subcutaneous fat.
The conventional methods of measuring body fat or subcutaneous fat, however, have several shortcomings. In particular, the output of the PD is affected by the skin thickness and color, as well as the thickness of the subcutaneous fat. The conventional methods, however, cannot and do not effectively compensate any errors attributable to the thickness and the color of the skin.